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Radioactivities from Stars to Solar Systems

Periodic Reporting for period 2 - RADIOSTAR (Radioactivities from Stars to Solar Systems)

Reporting period: 2019-03-01 to 2020-08-31

"We investigate the origin of our Solar System using radioactive nuclei as clocks to determine the time of occurrence of different astrophysical events before the birth of the Sun and the circumstances of such birth. We still do not know in which kind of environment the Sun was born. The birth cluster may have contained from few to several thousands Sun's siblings and be located within a small or a giant stellar nursery, perhaps rare types of stars were there nearby. Like for human beings, also for stars and their planetary system birth circumstances significantly determine their future life. Our objective is to contribute to the understanding of the question if the Solar System and the planets within it, and including our habitable Earth, were born under special circumstances (in which case we have been lucky) or if they constitute a normal, common type of planetary system in our Milky Way galaxy. To understand our own place in the Universe is one of the most fundamental quests of humankind. The immediate aim of our investigation is the interpretation of measurements provided by analysis of meteoritic rocks performed in the most advanced laboratories around the world. These experiments demonstrate that ""short-lived"" radioactive nuclei, with half-lives or the order of millions of years, were present at the birth of the Sun. We are calculating theoretical predictions of the abundances of these short-lived radioactive nuclei in the Milky Way to compare them to meteoritic data. In this way we can reveal the time line for the events that predated the birth of the Sun. The final aim is to identify the type of the stellar nursery where the Sun was born, which will also allow us to understand the still mysterious origin of the very short-lived radioactive nucleus: aluminium-26. This nucleus is crucial because the heat generated by its decay in the first few million years of the life of the Solar System affected the thermo-mechanical and chemical evolution of planetesimals and the delivery of water to the Earth. While we still do not know if its presence in proto-planetary disks is normal or special, we know that it is closely linked to circumstances of the birth of the Sun."
To predict the evolution of the abundances of radioactive nuclei in the Milky Way galaxy we need to understand: how are these nuclei produced by nuclear interactions in astronomical objects such as stars, supernova, and the mergers of compact objects, such as neutron stars? and how do their abundances evolve in the interstellar medium once they are ejected by these sources?

To answer the first question of the production of radioactive nuclei in stellar objects we have focused on many radioactive nuclei and the large variety of their possible stellar origins. We have calculated predictions for the production of aluminium-26 in the winds of massive stars (those roughly 10 times more massive than the Sun) also considering that most of these stars have a stellar companion of similar mass with which they interact. We are expanding these predictions to include the effect of stellar binarity also on other radioactive nuclei produced by winds, such as chlorine-36 and calcium-41. We have analysed the slow neutron-capture process that occur in giant stars (of initial mass a few times larger than the Sun), which are responsible for producing short-lived radioactive nuclei such as palladium-107 and hafnium-182, and established that stellar rotation, one of the major uncertainty of such models so far, should not have a major impact on the production of these nuclei. We have also considered the production of short-lived radioactive nuclei belonging to very heavy elements, such as curium-247, in rapid neutron-capture process sites (such as neutron star and black hole mergers) to identify the time when the last of such events contributed to the matter that formed the Solar System. Core-collapse supernovae, the final phases of the lives of massive star, also produce a large variety of radioactive nuclei. We are considering the production of fifteen such nuclei in these explosions using models that explore some of the many related uncertainties.

On the second question of the evolution of radioactive nuclei in the interstellar medium, we have published the first open-source computational code that follows the evolution of the abundances of the chemical elements in the galaxy including radioactive nuclei. This has allowed us to evaluate quantitatively and with error bars, the effect of galactic evolution and its uncertainties on isotopic ratios of radioactive to stable nuclei, the form in which the meteoritic data we compare our predictions to are obtained. We then moved onto considering that in the Galactic interstellar medium abundances can be heterogeneous, i.e. not perfectly mixed. This is particularly crucial in the case of radioactive nuclei because their decay time can be similar to the interval time between different injections from their stellar production sources into each specific location of the Galaxy where the Sun may have been born. We have developed a statistical method to evaluate quantitatively the effect of such heterogeneities and have started applying it to ratios of radioactive to stable nuclei, as well as of radioactive to radioactive nuclei. With this method we can evaluate not only one value for the absolute timescales related to the birth of the Sun, but also their error bars.
With our development of the modelling of the evolution of radioactive nuclei in the galaxy, coupled with the detailed analysis of their production in several types of stellar sources, we have now the most complete framework to date to investigate accurately the origin of the Solar System by exploiting radioactive nuclei and including uncertainties in the results. While we have so far considered the temporal heterogeneities of the stellar events, we are now developing tools to consider also the spatial heterogeneities of the stellar sources. This is under development from three different points of view using: 3D Galactic chemical evolution models that include the interaction of supernovae with the interstellar medium, a code that mimics propagation of matter in the interstellar medium via diffusion, and a full hydrodynamical code of galactic chemical evolution within a cosmological framework. While this analysis is still under development, we have started to apply it to promising cases of short-lived radioactive nuclei. Our very preliminary result is that the Sun should have been born roughly 9-13 million years after the formation of its stellar nursery. This means that the Sun was subsequently affected by the events that occurred within its stellar nursery for roughly this time interval. By the end of the project, we will give such information more confidently, and we will also have tested if this constraint allows us to produce a picture for the evolution of the stellar nursery and of birth of the Sun that explains the presence of aluminium-26 in the early Solar System.